Fixes #5965.

extensions/composer/Plugin.php

@@ -28,7 +28,7 @@ class Plugin implements PluginInterface
         $composer->getInstallationManager()->addInstaller($installer);
         $file = rtrim($composer->getConfig()->get('vendor-dir'), '/') . '/yiisoft/extensions.php';
         if (!is_file($file)) {
-            @mkdir(dirname($file));
+            @mkdir(dirname($file), 0777, true);
             file_put_contents($file, "<?php\n\nreturn [];\n");
         }
     }